1. Field of the Invention
The invention relates to a method for designing and/or optimizing a surgical device, to a method for manufacturing a surgical device, in particular an implant, to an implant manufactured by using said method, and to a method for fixing bones using said implant.
2. Description of the Related Art
Today surgical implants for fixation at or in a bone by means of bone fixation elements (e.g. screw or bolts) are manufactured taking into account the following parameters only    a) anatomical shape of the bone to or in which the implant is to be fixed;    b) type of fracture most frequently encountered; and    c) size of the patient.
No consideration is paid to the following parameters which are of even more importance than the above mentioned:    d) quality of the bone to or in which the implant is to be fixed (e.g. density, porosity, orientation of trabeculi and lamellae, distribution of cortex/spongiosa);    e) variation of these bone qualities depending on the specific patient (e.g. age, sex, race, size, health); and    f) post-operative loading conditions.
For the above reasons present designs of surgical implants, in particular of bone plates and intramedullary nails are mainly oriented to fracture typology and not to specific needs of a category of patients.
Thus, there remains a need for an improved method of design and manufacture of surgical implants for fixation at or in a bone by means of bone fixation elements.
It is an object of the invention to provide an improved method of design and manufacture of surgical implants for fixation at or in a bone by means of bone fixation elements which is more patient oriented, i.e. which takes into consideration additional relevant and critical bone parameters better defining each patient category.
The invention solves the posed problem with a method for designing and/or optimizing a surgical device as claimed, with a method for manufacturing a surgical device, in particular an implant as claimed, with an implant manufactured by using said method as claimed and with a method for fixing bone using said implant as claimed.
Definitions for currently used terms in the claims and the description:
General collection of three-dimensional bone quality data: a set of data including the entity of bone quality data analysed in the complete patient population, e.g. obtained from a variety of bone or bone portions.
Homologous sub-collection: the bone quality data of the N>2 homologous sub-collections is obtained from bone material of identical volume in identical anatomical positions of the patient population.
Categories: The patient population is divided in categories by applying the following criterions:
It is well known that nowadays surgeons do not consider all patients as belonging to the same category. For instance, osteoporotic and healthy individuals are considered as two different patient categories [Goldhahn J, Suhm N, Goldhahn S, Blauth M, Hanson B.; Influence of osteoporosis on fracture fixation-a systematic literature review; Osteoporos Int. 2008 June; 19(6):761-72. Review; and van Rietbergen B, Huiskes R, Eckstein F, Rueegsegger P.; Trabecular Bone Tissue Strains in the Healthy and Osteoporotic Human Femur; J. Bone Mineral Research, v. 18, N 10, p. 1781-1787, 2003]. The present method does not specify the criteria for patients' classification. The criteria defined to categorize the entire patient's population in subpopulations, should be medically and market driven. The method delivers a procedure to assess if the proposed categories are significantly different, if a different implant is required for each category, and how to design an optimized implant for the specific category.
Essential features but not limiting the chance of patients categorization are: age, health status, fracture pattern, bone mineral density, bone quality (microarchitecture, cortex/spongiosa ratio).
A patient is allotted to a particular category as follows:
The categorization should be medically and market driven, i.e. the surgeons and the companies will decide which patient is allotted to a particular class. The surgeons can be expected to potentially ask for a large number of categories and thus a large number of implants and the implant producer to restrict this number due to economical reason. Together with signalmen, health status and fracture pattern, information on bone quantity and quality are appropriate indexes for patient categorization. Currently, Dual Energy X-ray Absorptiometry (DXA) is the standard method to determine the osteoporotic state and to justify patient's treatment, e.g. with bisphosphonates. The present procedure is based on data collected with high-resolution peripheral Quantitative Computed Tomography (hr-pQCT), which provides, compared to DXA measurements, significantly more accurate and valuable information on bone microarchitecture (i.e. bone quality) and density. At the same time this technique is more time consuming and increases the X-ray dose for the patient, making the pQCT availability in hospitals significantly lower than DXA machines. Recent studies have demonstrated a good correlation between DXA and hr-pQCT in respect to bone density assessment [Grampp S, Lang P, Jergas M, Glüer CC, Mathur A, Engelke K, Genant H K; Assessment of the skeletal status by peripheral quantitative computed tomography of the forearm: short-term precision in vivo and comparison to dual X-ray absorptiometry. J. Bone Miner Res. 1995 October; 10(10)1566-76]. Based on these facts, it is believed that it will be possible to assess the relation between categorizations based both on DXA and on hr-pQCT. Once the relation will be demonstrated DXA measurements might also be used as an additional tool to allot a patient to a certain category.
To correctly and reproducibly position the implant on or in the patient's bone or virtually on or in the bones of the database the following procedure can be applied:
In the clinical setting the space available to position an implant is substantially limited by the given surgical access. Anatomically shaped implants limit even more the chances for positioning. Therefore, the surgical approach described in implants' manuals allows reproducible positioning of an implant. The present method is dependent on the surgeon's ability in positioning the implant at the right location, however, the present method allows to quantify the precision of this positioning and its effect on the regions investigated.
The surgeons follow the described technique for surgical accesses. These take into consideration anatomical landmarks and boundary conditions of the bone (e.g. musculoskeletal structures, vessels, nerves). The implant anatomical shape gives an additional restrain to the implant positioning helping the surgeon finding the right location.
Mechanical properties of bony structures: The mechanical properties of the bony structures (e.g. bone strength) depend on many factors. In the bone research/industrial community it is well accepted that bone mineral density and distribution as well as bone microarchitecture are the most important contributing factors. It is known that cut-out risk after fracture fixation depends on the load transfer between implant's anchoring elements and bone fragments. In vitro mechanical testing demonstrated a direct relationship between mechanical properties (e.g. bone strength and failure behaviour) and bone's macro- and microproperties (e.g. bone density, trabecular structure) [Hildebrand T, Ruegsegger P.; Quantification of bone microarchitecture with the structure model index. Comp. Meth. Biomech. Biomed. Eng. 1: 15-23, 1997. Ulrich D, van Rietbergen B, Laib A, Rüegsegger P.; The ability of three-dimensional structural indices to reflect mechanical aspects of trabecular bone. Bone 25 (1): 55-60, 1999. Gabet Y, Kohavi D, Voide R, Mueller T L, Müller R, Bab I.; Endosseous Implant Anchorage is Critically Dependent on Mechanostructural Determinants of Peri-Implant Bone Trabeculae. J Bone Miner Res. 2009 Aug. 4. Hernandez C J, Keaveny T M.; A biomechanical perspective on bone quality. Bone. 2006 December; 39(6):1173-81].
The most important parameters used to define trabecular bone structure are:                Trabecular Thickness        Trabecular Separation        Trabecular Number        Trabecular Bone Pattern Factor        Euler number, indicator of connectivity in a 3D structure        DA degree of anisotropy        SMI (structure model index) which defines the prevalent trabeculae shape        Mean polar moment of inertia which indicates the resistance to a rotation of a cross section about a chosen axis        Porosity        
High quality volumes/Low quality volumes: High quality volumes, namely those suitable for anchorage of a bone fixation element, are defined as those where, due to the combined effect of bone quality/quantity and loading pattern, the resulting stress accumulation is minimal.
Low quality volumes, namely those where the operator might wish to increase the implant purchase and/or to augment the physical properties of the bone structure, are those where, due to the combined effect of bone quality/quantity and loading pattern, the resulting stress is critical.
The definition of maximum and minimum, or low and high quality volumes, determined inside each category based on statistical analyses.
Optimal place/optimal direction: Includes the loading pattern as a factor helping to assess where the implant best purchases. The whole set of variables will be tested with computational and biomechanical models.
Additionally, the through hole in the implant should be easily accessible and should respect the local anatomy. For example, in the humerus the through hole can not start on the lateral bone surface, exit in the bicipital groove and enter again medially in the humerus. The boundary conditions given to the present algorithm will exclude those directions and find among the available directions those where the quality of the volumes is optimal. It is to be clarified that the algorithm leaves the chance to augment also in those areas where the implant has already good purchase, i.e. not only low quality for augmentation and high quality for better implant purchase.
Configuration of the through holes: For a given category of patients and implant, the method optimizes the combinations of all the available anchorage parts. The optimization is completed when the combination of the investigated high quality volumes results in the minimal stress accumulation taking in consideration all the anatomical, technical (superimposition of screws) and surgical limitations.
At the same time for a given category of patients and implant, the method optimizes the combinations of all the available through holes dedicated to aim low quality volumes.
The number of through holes dedicated to aim low quality volumes is set according to the optimized number of anchorage parts and to final optimization concerning the best stress shearing configuration between bone and implant, also taking in consideration all the anatomical, technical (superimposition of screws) and surgical limitations. According to patients categorization the combination of low and high quality volumes can be identical or not and it aims to create minimum bone defects and maximum fixation stability. The method determines the best combined configuration of the directions of the axes of any locking system optimised for any purpose on any medical implant dedicated to any specific category of patients.
The depth of the bore hole in the bone or in particular the depth of the location for the application of bone cement taken into account by the surgeon when implanting said surgical implant can be determined as follows: Nowadays surgeons use fluoroscopic images to assess how deep in the bone they drilled. The present method gives the surgeon the tool to find the direction of the weakest region. Actual or future imaging techniques can be used to assess the holes depth.
In a special embodiment of said method said optimal place and said optimal direction is defined by the position and extension of bone volumes where the stress accumulation resulting from a combination of bone quality and load pattern is minimal. The summarized stress accumulation for all through holes of the implant is the criterion for implant optimization, design and manufacture with regard to the position and direction of each through hole.
In another embodiment of said method said optimal place and said optimal direction is defined by taking into account the position and extension of augmented low quality volumes, preferably augmented by means of applying a bone cement. A new generation implant can be manufactured according to one or both the above optimization criteria, as in the following examples:                An existing implant can be optimized with angular stable holes allowing precise local augmentation.        The direction of the anchoring parts of an existing implant can be optimized according to the high quality volumes characteristics of a given category, by maintaining the existing positions of the through holes.        The direction of the anchoring parts of a new implant can be optimized according to the high quality volumes characteristics of a given category, defining new positions of the through holes in an existing implant shape or in a newly designed implant shape.        The direction of the anchoring parts of a new implant can be optimized according to the high quality volumes characteristics of a given category, defining new position of the through holes in an existing implant shape or in a newly designed implant shape and with angular stable holes allowing precise local augmentation.        
In again another embodiment of said method said optimal place and said optimal direction is defined by additionally taking into account the accessibility of the through hole through the bone tissue.
In a further embodiment of said method said implant is a fixed angle implant. In fixed angle implants the directions of the anchoring parts are defined by the through holes. These are characterized by a mechanical locking method which allows consistently pre-drilling and inserting anchoring parts along the same through holes orientation.
In yet a further embodiment of said method high resolution bone quality data is used allowing the assessment of the bone micro architecture at a resolution smaller than or equal to 100 μm. To assess bone micro-architecture, which is relevant for implant optimization, high resolution CT is necessary. Clinical CT cannot deliver such information. The present method provides the assessment of the bone micro architecture via CT scans performed at a resolutions 100 μm (intra-trabecular space).
In a special embodiment of said surgical device, in particular implant said central axis has a direction that, respecting the anatomical restrains, allows following the best combination of purchase directions for all the implants anchorage elements.
In another embodiment of said surgical device, in particular implant the orientation of said central axis and the position of said through holes are defined according to a relative reference system.
In case of an implant configured as a bone plate said relative reference system is given by the center of three not aligned monocortical holes, drilled as anchorage for the implant fixation points. In case of an implant configured as an intramedullary nail said relative reference system is given by the coordinate system created by the center of the three non aligned monocortical holes, drilled as anchorage for the implant fixation points. In case of a general medical device said relative reference system is given by the coordinate system created by the center of the three non aligned points, used as anchorage for the device fixation points.
In still another embodiment of said surgical device, in particular implant said through hole is provided with coupling means for temporarily attaching an instrument.
In a further embodiment of said surgical device, in particular implant said through hole is designed as a fixed angle hole for receiving a bone fixation element at a fixed angle.
In yet a further embodiment of said surgical device, in particular implant said through hole (3) is designed as a variable angle hole for receiving a bone fixation element at a variable angle, said variable angle hole having a central axis (4) corresponding to said optimal direction (see FIG. 1). This configuration allows the advantage that the variability of the angle allows the surgeon to align the bone fixation element upon insertion to take into account e.g. anatomical differences.
According to a further aspect of the invention an assembly is provided which comprises the surgical device, in particular implant according to the invention and an aiming or drilling device attachable to said coupling means.
In accordance with another aspect, an assembly comprising the implant according to the invention and a bone replacement material source attachable to said coupling means is provided. A bone augmentation technique with any material can be applied. By means of said source, e.g. a syringe or cannula it is possible to introduce bone replacement material at the weakest region of the bone.
In a special embodiment of said method for fixing fractures bones the fracture is reduced in a first step before applying said implant.
In accordance with yet another aspect a method is provided for selecting an optimal implant out of said designed or optimized implants for each of said N>2 categories of patients.